With recent advances in cell transplantation, tissue engineering and genetic technologies, the living cell is becoming an important therapeutic tool in clinical medical care. From the use of living artificial skin and bone material to treat burn and trauma victims, to bioartificial devices and direct transplantation of cellular material to treat the increasingly long list of genetically-based diseases, living cells are increasingly incorporated into comprehensive treatment. In such a construct, the exogenous cells perform the multitude of complex tasks which the diseased tissue cannot. Successful long-term preservation and storage of mammalian cells is critical to the success of this type of medical care.
Conventional cryopreservation protocols rely on the addition of cryoprotectants to control the formation of damaging crystalline ice in the intracellular and extracellular liquid. The formation of ice in the extracellular liquid leads to dehydration of cells and has also been shown to catalyze the formation of intracellular ice [Toner et al., "Thermodynamics and kinetics of intracellular ice formation during freezing of biological cells," 67 J.Applied Phys. 1582-1593 (1990)]. The formation of intracellular ice directly damages the cell and usually leads to cell death.
Equilibrium and non-equilibrium cryopreservation protocols try to balance the deleterious effects of cell dehydration, exposure of the cells to toxic cryoprotectants and the lethal formation of intracellular ice so as to yield the highest possible percentage of viable cells [Mazur, "Equilibrium, Quasi-equilibrium, and non-equilibrium freezing of mammalian embryos," 17 Cell Biophysics 53-92 (1985)]. Protocols of this type have been very successful for certain cell types. For example, survival rates of greater than 90% have been reported for erythrocytes [Nei, "Freezing injury to erythrocytes I. Freezing patterns and post-thaw hemolysis," 13 Cryobiology 278-286 (1976)], pancreatic islets [Jutte et al., "Vitrification of Human Islets of Langerhans," 24 Cryobiology 403-411 (1987)] and mouse oocytes [Karlsson et al, "Fertilization and development of mouse oocytes cryopreserved using a theoretically optimized protocol," 11 Human Reproduction 1296-1305 (1996)].
There are many cell types, however, for which acceptable cryopreservation protocols have not been developed. This is largely due to the fact that the concentration of cryoprotectant required to avoid intracellular ice formation is too high for most cells to tolerate [Fahy et al, "Vitrification as an approach to cryopreservation," 21 Cryobiology 407-426 (1984)]. Among the important cell types for which successful and reliable freezing protocols have not been developed are hepatocytes, human oocytes, platelets and granulocytes. The fact that human oocytes have not been preserved successfully in spite of the successful freezing of mouse oocytes illustrates another difficulty with the current methods of cryopreservation: they are extremely dependent on cell type. Even closely related cell types behave and survive differently when cryopreserved.
An alternative to conventional approaches to cryopreservation by freezing with high levels of cryoprotectant is vitrification, i.e., solidification of a liquid into an amorphous or glassy state as opposed to the crystalline state. Unlike the liquid-to-crystal transition, the liquid-to-glass transition is generally believed not to have any adverse biological effects. This is because there is no elevation in electrolyte concentration, no ice crystals to cause mechanical damage, and no potentially damaging osmotic shifts during the vitrification of cell suspensions.
It appears that nearly all liquids would undergo a transition to a glassy state if crystallization is bypassed on cooling. A necessary and sufficient condition for this transition is that the liquid solution should be rapidly cooled to the glass transition temperature so that nucleation and crystal growth cannot occur. Typically, the requisite cooling rates are very high for water (approximately 10.sup.7.degree. C./min), but they can be reduced to more workable levels (approximately 10.degree. C./min) by the addition of cryoprotectants (CPA, usually 50 to 60% w/w). However, CPA concentrations this high are typically lethal to biological cells. New methods of ultra-rapid cooling are needed to achieve glassy state during cooling of biological cell suspensions.
The formation of intracellular ice during freezing may be avoided by hyperquenching the cells. In hyperquenching the water is cooled so quickly that nucleation events do not occur and the liquid undergoes a glass phase transition. As liquid water is cooled below its freezing point, it becomes energetically favorable for nucleation to occur. At 130K, however, liquid water goes through a glass phase transition, which is a second order thermodynamic phase transition, and the relaxation time for the molecules becomes greater than laboratory time scales--i.e., the viscosity of the fluid increases so that molecular rearrangement into crystals becomes impossible. If one can get water to the glass phase before ice crystals nucleate, then one creates an amorphous solid referred to as either amorphous solid water or amorphous ice.
There are a number of ways to form glass phase solid water; water vapor deposition on to cryo-cooled plates at very low pressures, exposing crystalline ice to very high pressures at temperatures below 130K and thereby crushing it to the glass phase, and spraying water micro-droplets at supersonic velocities onto cryoplates. None of these techniques, however, is suitable for use in freezing cells. Water vapor deposition simply cannot be done with a cell, and the other two methods expose the cell to lethal stress.
In order to preserve the wide variety of cellular material needed in current medical procedures, a new method of ultra rapid cooling that can successfully cryopreserve biological material without the formation of intracellular ice is needed. Such a method must avoid the use of high concentrations of CPAs which are toxic to many cell types and not expose the biological material to damaging physical stresses.